The SS communication method has as characteristics that it is resistant against disturbance, noise and fading and it has a signal concealing property and secrecy, and that asynchronous random access is thereby possible, as described in "Spread Spectrum Systems" by R. C. Dixon.
In such an SS modulation communication, the data modulation-demodulation system suitable for this communication method was heretofore a problem to be solved.
For this reason the applicant of the present application has proposed a new data modulation-demodulation method in two older Japanese patent applications Hei 1-29538 and Hei 1-244931. By the data modulation-demodulation method disclosed in the first of the two Japanese patent applications described above, on the transmitter side an SS code is transmitted after having been formed by switching between a plurality of pseudo noise (hereinbelow abbreviated to PN) codes different from each other according to values of data bits, while on the receiver side a surface acoustic wave (hereinbelow abbreviated to SAW) convolver is used as a correlator so that data are demodulated without requiring any synchronization between a PN code included in a received signal and a reference PN code generated within the receiver, as indicated in FIGS. 1 and 2 thereof. The system described in this patent application is so constructed that the reference PN code therein tracks the PN code included in the received signal, as indicated in FIG. 9 thereof. Consequently it is devised so as to always obtain a correlation output. On the other hand, the system disclosed in the latter (Japanese patent application No. Hei 1-244931) of the patent applications described above is an improvement of the system disclosed in the first, in which a pattern matching circuit and a low pass filter are added to the output side of the correlator to reduce influences of noise and disturbance so that detection of the presence or absence of the correlation output is made surer and that the tracking method, by which two PN codes are switched-over at detecting disappearance of the correlation output, can be effected more efficiently.
However, in the latter patent application (i.e. patent application No. Hei 1-244931), since in-loop delay of the tracking loop is increased if the performance of the added circuits is increased, which can give rise to out-tracking, it was difficult to obtain performance higher than a certain degree.
Therefore, the applicant of this application has proposed in the patent application No. Hei 2-331399 an SS communication method and a device capable of demodulating data more surely without using any tracking loop.
By the SS communication method according to the invention in the older patent application described above, on the transmitter side (a) a first PN code train consisting of first PN codes repeated with a predetermined period and a second PN code train having a phase deviated from the phase of the first PN code train by a predetermined value are generated, and (b) an SS modulated output is generated by selecting either one of the first and the second PN code trains according to each bit of the data to be transmitted so that each bit of the data to be transmitted is CPSK-modulated. On the other hand, on the receiver side, (a) a correlation-demodulated output is generated by correlation-demodulating a received input with a third PN code train consisting of PN codes repeated with the predetermined period described above, which are inverted in time with respect to the first PN code stated above, and (b) the correlation-demodulated output described above is CPSK-demodulated. This CPSK demodulation is effected (i) by generating a first time window pulse train consisting of pulses synchronized with the correlation-demodulated output described above and having a period which is equal to a half of the period stated above, by generating a second time window pulse train consisting of pulses deviated by the predetermined phase with respect to the pulse train described previously, and by generating a bit period indicating signal indicating the period of each bit of the data to be transmitted described previously, (ii) by generating a binary converted pulse output obtained by binary-converting the correlation-demodulated output with a predetermined threshold, (iii) by detecting a difference between the number of pulses within the first time window and the number of pulses within the second time window described above in the binary-converted pulse output in the period of each bit, and (iv) by determining the state of each bit, depending on the polarity of the difference described above.
FIG. 5 is a block diagram indicating a device for realizing the SS communicating method according to the invention disclosed in the older application described previously, using a SAW convolver for the correlator in the receiver and CPSK for the modulation-demodulation system.
In the figure, T represents a transmitter, in which reference numeral 1 is a PN code generator; 2 is a multiplexer (MPX); 3 is a shift register; 4 is a double balanced modulator; 5 is a HF signal generator; and 6 is a transmitting antenna. On the other hand, R represents a receiver, in which reference numeral 7 is a receiving antenna; 8 is a HF amplifier-frequency converter-IF amplifier; 9 is a correlator (e.g. SAW convolver); 10 is a PN code generator; 11 is a HF signal generator; 12 is a detecting circuit; 13 is a voltage comparator; 14 and 15 are AND gates; 16 is a demodulating circuit including an up-down counter; and 21 is a double balanced modulator.
In the transmitter T, as indicated in FIG. 6, the first PN code (i) having the predetermined period generated by the PN code generator 1 is inputted directly to the MPX2 and at the same time the second PN code (ii) obtained by delaying the first PN code by a predetermined phase by the shift register 3 is inputted in the MPX2. The delay amount of the first PN code by the shift register 3 is determined in accordance with the phase shift amount of the code at the CPSK demodulation. Concretely speaking, it is selected so as to be about 1/2 of a period of the PN code. That is, denoting one period of the PN code by T.sub.PN, about 1/2T.sub.PN is selected for the delay amount. (ii) in FIG. 6 represents the state thereof. This is because the phase difference between the two codes (in this case PN1 and PN2) when the CPSK modulation is effected is maximum so that the number of phase judgment errors at the demodulation can be minimized, if the delay amount is selected as described above.
Data are given to the MPX2 as a control input (iii), and a PN code (iv) having a different code phase is outputted to the modulator 4 in response to these data. The modulator 4 multiplies the PN code (iv) stated above by an HF signal to output an SS modulated signal (v), which is transmitted through the antenna 6.
On the other hand, in the receiver R, a received signal (vi) coming from the antenna 7 is amplified and frequencyconverted by the circuit 8 and a signal (vii) thus obtained is inputted to one of the inputs of the correlator 9. On the other hand, a reference signal (viii) is applied to the other input of the correlator 9.
This reference signal (viii) is generated by multiplying a third PN code (ix), which is inverted in time with respect to the first PN code (i) coming from the PN code generator 10, by an HF signal (x) in the modulator 21. The SAW convolver disclosed in the older application No. Hei 2-331399 described previously, as indicated in FIG. 8, can be used for the correlator 9. Here the received signal s(t) and the reference signal r(t) are received by a terminal IN1 and the other terminal IN2, respectively, and inputted thereto through interdigital transducers IDT8. 83 is a gate electrode for taking out the output; 84 is a zinc oxide (ZnO) layer; 85 is a silicon oxide (SiO.sub.2) layer; 86 is a silicon (Si) layer; and 87 is an ohmic electrode. In the case where a SAW convolver is used for the correlator, since the input directions to the convolver are opposite to each other for the PN code and the reference code, one of the signals which are to be correlated with each other should be inverted in time. In this embodiment, the third PN code is inverted in time with respect to the first PN code.
A correlation output (xi) consisting of HF signals having a spike-shaped envelope part, as indicated in FIG. 7, is generated by the correlator 9. This spike-shaped part is a self correlation peak, which is called a correlation spike, in the case where the PN code in the received signal and the PN code in the reference signal are in accordance with each other.
The generator interval between two adjacent correlation spikes, in the case where the signal is not modulated with data, is equal to 1/2 of the period of the PN code in the received signal (which is equal to the period of the PN code in the reference signal). On the contrary, in the case where the received signal is CPSK-modulated, as described previously, the generation interval between two adjacent correlation spikes varies, depending on the data, as indicated by (xi) in FIG. 6.
The correlation output (xi) is envelope-detected by the detecting circuit 12 and the detection output thereof is given to one of the inputs of the comparator 13. A predetermined reference voltage is inputted to the other input of the voltage comparator 13 and a binary converted correlation pulse (xii) is obtained at the output thereof.
This correlation pulse is applied to one of the inputs of each of the AND gates 14 and 15 and time window pulses (xiii) and (xiv) are applied to the other inputs, respectively, after having been generated by a burst synchronization, as indicated e.g. in the Japanese patent application No. Hei 2-331399 described previously. Therefore, the correlation pulse passes through either one of the AND gates only during a period of time where either one of the time window pulses exists and is applied to the demodulating circuit 16.
The demodulating circuit 16 up- and down-counts pulses passing through the respective gates by means of an up-down counter included therein, in order to judge during which time window more correlation pulses pass therethrough to effect the data demodulation. Further, the time window pulses described above are synchronized with the correlation output, and e.g. the burst transmission method described in the older application stated previously, i.e. patent application Hei 2-331399, may be used for the synchronizing method.
Although the system of the SS communication device disclosed in the older application described above is basically excellent, it was found that it still had problems described below when it was used in practice.
For example, in the case where a plurality of narrow band disturbing signals are mixed in a received signal, depending on the relation between these disturbing signals and the frequency used in the SS communication device, a strong disturbing correlation output appears in accordance with the generation period of correlation spikes due to the object signal, as indicated by (b) in FIG. 7. In the case where both the object signal and the disturbing signals exist, the correlation output can be as indicated by (c) in FIG. 7. In this case, although the correlation spike due to the object signal is in accordance with the time window pulse (xiii):(c-1), since the disturbing signals are in accordance with the other time window pulse (xiv):(c-2) and in addition the disturbing signals are stronger, the number of binary converted correlation pulses generated by the disturbing signals in accordance with the time window pulse (xiv) is greater. Consequently, as the result, errors are produced in the demodulation by the up-down count of the correlation pulses described above.